LSIs have had a remarkable increase in signal transmission speed. Now, 10-Gbps transmission is available. Moreover, research is under way, aiming at 40-Gbps or 100-Gbps transmission speed. The 40-Gbps or 100-Gbps transmission speed is in the range of microwave rather than in the range of electric circuits. Digital signals are currently transmitted in the form of a square wave, but in reality the shape is close to a sine wave. However, depending on the signal pattern, low frequency components may be included. When the transmission speed becomes faster, the amount of frequency components to be transmitted should be small because it is easy to perform impedance matching and to make transmission characteristics better. Since a microwave circuit modulates carrier waves to transmit signals, the bandwidth of the signals is far narrower than that of the current digital signals. It is expected in the future that like the microwave circuit, the LSI will modulate carrier waves to transmit digital signals. In such a case, a band pass filter needs to be formed on a LSI printed board.
In the range of microwave circuits, the impedance varies according to the length, and there is no clear classification like inductance (L) or capacitance (C). As shown in FIGS. 55, 56 and 57, a band pass filter (BPF) is formed with the help of resonance on the length of which is λ/4 (λ represents wavelength) or the like. In FIG. 2.50 on page 57 of Non-Patent Document 1, the configuration of the BPF in FIG. 55 is illustrated and in FIG. 3.19 on page 112, the configuration of the BPF in FIGS. 56 and 57 is illustrated.
In FIG. 55, a planar BPF is formed with the use of microstrip lines (MSLs). The microstrip lines 120 overlap each other by λ/4. In FIGS. 56 and 57, a three-dimensional BPF is formed with lines formed in high dielectric ceramics. In a boxy ground 224 including the upper surface of which does not have a conductor while the other surfaces have conductors, two stub conductors 221 and 222 are formed. The lower ends of the stub conductors 221 and 222 are connected to the lower surface of the boxy ground conductor 224. The box is filled with a dielectric material 223.
As a technique relevant to the present invention, Patent Document 1 describes the structure of a board for high-speed transmission suitable for differential transmission, where a ground plane is disposed around a via for impedance matching. Patent Document 2 describes a multilayer circuit board where a signal line passes through between two non-conductive areas in a plane that exists on a different layer from a layer having the signal line.    Patent Document 1: JP-A-2005-277028    Patent Document 2: JP-A-2005-116544    Non-Patent Document 1: “Design and application of a filter circuit for communication,” Yoshihiro Konishi, Sougou Denshi Syuppan, Apr. 5, 1997, Second edition, pp. 57 and 112